The invention relates to an ultrasonic anemometer for measurement of the velocity of inflowing wind with at least one transmitter for emitting sound waves and at least one receiver for at least partially receiving the emitted sound waves, and with an evaluation unit, which, taking a recorded transit time of the sound waves on a measuring section located between transmitter and receiver as the basis, determines the quantity of at least one component of the wind vector and/or the velocity of sound.
Different measuring instruments are known, using which the local measurement of the velocity of a flow field, in particular of the wind velocity, is undertaken. A special kind of wind measuring devices or so-called anemometers, resp., are ultrasonic anemometers. Ultrasonic anemometers, which have been known for a long time, use the principle of measurement of the sound wave's transit time between transmitter and receiver. Here, it is utilized that sound waves are carried along by the medium in which they propagate, so that the transit time of signals across a measuring section with a fixed length depends on the flow through the measuring section. Using sound waves with high frequency or high bandwidth, resp., transit times can be determined particularly accurate, so that on measuring sections with a short distance, high-frequency sound waves are preferably used. Since the velocity of sound depends on air temperature as well as on air humidity, usually transit times are determined in both directions, i.e. bidirectional. Furthermore, from the sum of these two transit times, the so-called virtual temperature can be calculated.
Known ultrasonic anemometers usually have several measuring sections between the individual ultrasonic transmitters and receivers, via which the velocity of sound is measured in various directions in space. From the measured values determined, electronic measuring equipment calculates the horizontal and the vertical wind velocity.
For measurements of the three wind components, in particular for measurement of the respective average values, and the velocity of sound as well as their turbulent fluctuations in the atmosphere, ultrasonic anemometers in various embodiments are used, as stated, for example, in VDI Guideline 3786 Sheet 12. Depending on the sensor head design, one, two or three measuring sections are used. These are formed by sound transducers serving as transmitter and receiver, which are located at the ends of the measuring sections and send and/or receive sound signals along the measuring sections. A substantial criterion for the arrangement of the measuring sections and the sound transducers is the minimization of measurement errors due to flow deformations or shadowing, resp., by the sound transducers themselves. The error occurring is highest, when the inflow direction is parallel to a measuring section, and smallest, when the inflow direction is perpendicular to the measuring section.
In this context, an ultrasonic anemometer is known from DE 689 01 800 T2, using which the transit times of sound waves on various measuring sections between the individual ultrasonic transducers are recorded and evaluated. The ultrasonic anemometer described has an arrangement of emitting and receiving ultrasonic transducers, which are arranged such that they define at least three different ultrasound transmission paths in the air. Furthermore, electronic measuring equipment is provided, so that, on the basis of the measurement of propagation times of the ultrasonic waves along the various paths, the wind direction as well as the wind velocity are determined considering the measured propagation times.
In general, with the ultrasonic anemometers usually used, two different types of measuring section arrangements are used. For a first type of sensor head, one measuring section is arranged vertically and two measuring sections are arranged horizontally, whereas for a second type of sensor head, three measuring sections are typically inclined by an angle of 45° to 60° and their relative azimuth angles are 120°.
The common setup of the ultrasonic anemometers considering the transit time is thus based on the arrangement of antiparallel propagation paths, on which the transit time of the sound is measured. For that, ultrasonic transducers are respectively required at both ends of each measuring section, which preferably work alternately as transmitter and as receiver. Usually, reciprocal sound transducers are used today, which respectively combine the transmitting and receiving functions.
A frequent purpose in the determination of the three wind components, in particular of the respective average values, and the velocity of sound, including the turbulent fluctuations in the atmosphere, is the determination of vertical so-called “eddy covariance” flows of air admixtures and energy, as they are part of international measurement programs, as e.g. AmeriFlux, EUROFLUX, and Mediflux. In particular, heat, water vapor, carbon dioxide and methane flows are monitored in this manner.
There are high accuracy requirements for such measurements, above all for the measurement of the vertical wind component. Measuring locations are preferably chosen such that the wind vector, on average, is directed almost horizontally.
In this context, the sound transducer arrangement of the first type first described above, with one vertical and two horizontal measuring sections, due to the vertically arranged measuring section, with almost horizontal inflow, directly provides the vertical wind component, which under these conditions is afflicted with a particularly small error by shadowing effects. A substantial disadvantage of this arrangement is the horizontal orientation of the other measuring sections. Hereby, the usable wind direction area is limited, since sectors with an inflow direction almost or completely in parallel with the section have a high shadowing error and result in a respectively lower quality of the measured horizontal wind components. Though the vertical section enables high measuring accuracy of the vertical wind component, one disadvantage of this section orientation is wetting of the lower sensor by rain or dew or, in case of heated sensor heads, by melt water. This effect can be explained by the transformation of electromechanical vibrations into sound vibrations of the air taking place via a vibrating surface or a membrane, resp., wherein the sound energy is preferably radiated vertical to the membrane. Therefore, the membranes of the vertical measuring section are oriented horizontally, which results in the fact that rain- or dewdrops may collect on the lower membrane and result in an interference with or even interruption of the measurement. For the upper sensor, a similar situation results, since due to rain, dew or, in particular with heated sensor heads, melt water, drops are formed, which then adhere to the sound transducers and partially or completely cover their surface.
With a sensor arrangement according to the second type with inclined measuring sections, the disadvantages described above are avoided, so that respectively designed ultrasonic anemometers are widely used in EC measurement programs. It is, however, disadvantageous, that the vertical wind component must be determined from the measurements along the inclined measuring sections. Thus, shadowing effects on these measuring sections influence the accuracy of the derived vertical wind component. More recent examinations have shown that with the inclination angles used, due to the shadowing effects, a significant underestimation of the vertical wind component and thus of the EC flows occurs. It is furthermore disadvantageous for the accuracy of the vertical component, that this must be indirectly determined by combination of the three inclined components.